In the disk array system, there is generally used a method, by which the data of a faulty data drive are saved in a spare drive packaged in advanced for an alternate. When the system is working in a sound state, the data of the data drive need not be saved in the spare drive so that the spare drive itself is not utilized in the least. The number of drives to be packaged in the disk array system is so limited that it cannot be made large at random.
JP-A-2003-186630 has provided a method for utilizing a small number of spare drive areas efficiently. In case a spare drive has a capacity larger than the individual ones of a plurality of data drives, according to that method, the spare drive is divided into a plurality of partitions, in which the data of the individual data drives are individually saved. Alternatively, in case a data drive has a larger capacity than the individual ones of a plurality of spare drives, the data drive is divided into a plurality of partitions, the data of which are individually saved in the individual spare drives.
A method described in Patent Publication 1 for saving the data in the spare drives can use the capacity of the spare drives to some efficient extent. So long as the individual capacities of all the data drives and all the spare drives are not equal, integral times or one integral number-th, however, a spare empty area may occur in the spare drives. In case the data of two data drives of 30 GB are already saved in a spare drive of 70 GB, for example, an empty area of 10 GB is left in that spare drive. In case the data of another data drive of 30 GB are then to be saved, the empty area of 10 GB is too small to use. Therefore, another spare drive is employed to leave the empty area of 10 GB useless.
The drives to be packaged in the disk array system have a variety of capacities so that the aforementioned problem occurs highly probably. In order to minimize the spare area, moreover, it is difficult in fact to decide what spare drive the saving area of a data drive is to be assigned and what area of the spare drive the same area is to be assigned.
Let the case be considered, in which a plurality of data drives of 30 GB, a plurality of data drives of 40 GB and two spare drives of 60 GB and 70 GB exist together, for example. When the data of a data drive of 30 GB are to be saved, whether it is optimum or not which of the spare drives of 60 GB and 70 GB is to be selected is different depending upon which of the data drives of 30 GB and 40 GB is to be saved in the future. Moreover, this decision is unknown at this stage, and it is seriously difficult to select the optimum assignment.